Pulse compression involves the rearrangement of the temporal distribution of energy in a Radio Frequency (RF) pulse in such a way that a long transmitted pulse with a given energy is transformed to a shorter received pulse with the same energy. Since the total energy is the same for both, the instantaneous power during the shortened pulse is therefore greater than the instantaneous power during the long pulse. Pulse compression is useful in radar systems for improving range resolution and discrimination of unwanted signals such as radar clutter and multiple targets. In active sensors pulse compression allows long-pulse, limited peak power systems to achieve performance equivalent to a shorter-pulse higher-peak power system. Ideally, pulse compression is implemented with matched filters where the processing device is a network with impulse response matched to the time reverse of the long-pulse waveform. This matched-filter operation results in maximizing the signal-to-noise ratio and in optimum detection of the target.
Many waveforms have been used for pulse compression, including coding of the phase of a carrier signal. Bi-phase modulation using binary codes is one common approach with Barker Codes being popular. Other binary waveforms that have been used for pulse compression include pseudo-random codes and random binary codes. Nonbinary waveforms that have been use for pulse compression include FM modulation signals and polyphase codes.
A problem that has limited the utility of pulse compression and correlation receivers in radar system has been the existence of temporal/range sidelobes in the correlation function of the radar waveform. These sidelobes allow out-of-range-gate returns, such as clutter, to compete with a target in a particular range gate of interest. A number of research efforts have addressed this problem in the past, and several waveform designs have resulted in the potential reduction or elimination of the range sidelobe problem. For example, Barker codes are known for lengths only up to N=13, and they do not match the desired "perfect" range correlation property.
Application of Golay code pairs (also known as complementary sequences) involves processing two coded pulses at a time in a radar processor to eliminate the range sidelobes. These codes have the property that when their individual range sidelobes are combined (algebraic addition), the composite sidelobes completely cancel, yielding the desired perfect correlation property.
Pulse compression, through a new techinque, has been revealed (U.S. Pat. No. 4,472,717) which utilizes the polarization of the expanded and radiated signal as the vehicle of coding to achieve compression of the received pulse. This is a unique approach which offers a doppler insensitive waveform, avoiding the mismatch losses suffered in typical pulse compression methods.
Most pulse expansion/compression techinques result with undesirable time-sidelobe responses which require amplitude weighting (windowing) to achieve acceptable sidelobes. A pulse encoding and processing technique has been developed (U.S. Pat. No. 4,513,288) to achieve zero time sidelobes, this approach being called Group Complemetary Coding. The combination of Group Complementary Coding and polarization pulse compression is expected to offer enhanced exploitation of target and clutter polarization characteristics for discrimination between the two.
This disclosure presents definition of the two techniques and addresses combining the two for the benefit of achieving pulse compression with zero time sidelobe responses and improving polarization isolation between orthogonally polarized channels.